1. Field Of The Invention
The present invention relates in general to an improvement of a field winding assembly provided in a rotor for an electric rotary machine and in particular the invention is concerned with an improvement of insulators for winding conductors mounted in winding slots formed in the rotor core.
2. Description Of The Prior Art
As is known in the art, in the case of an electric rotary machine, the field winding assembly for a rotor is composed of a plurarity of windings each being formed of a stack of conductors and mounted into respective one of slots formed in the outer peripheral portion of a rotor core and extending radially outwardly. With a view to preventing creep current from flowing to the rotor core from the winding conductors, an insulating layer is provided so as to enclose the whole stacked winding conductors in each slot in a continuous manner. For example, in the case of a rotary machine of relatively small capacity, the stacked conductors in each slot are wound in a continuous manner with an insulation tape or sheet formed of insulating material such as mica, asbestos or the like. However, in the case of the rotary machine such as a turbine generator whose capacity has been increased over the years, the windings thereof are apt to undergo great thermal expansion and vibrations and to be subjected to a large centrifugal force. In view of such adverse conditions, the insulating layer provided between the rotor core and the conductors must be capable of well withstanding such adverse conditions. In particular, as the capacity or output of a rotary machine is increased, deformation of the winding conductor due to thermal expansion and contraction thereof produced upon repeated starting and stopping operations in the running of the rotary machine will become greater on the one hand, while the insulating layer will not be subjected to any appreciable deformation on the other hand, thus involving relative sliding or abrasive movements between the conductors and the surrounding insulation layer. As a result, the insulating layer will suffer from deformation or damage, which will eventually lead to generation of cracks after long the insulating layer, in operation of the rotary machine. In this connection, it will be noted that the danger of the insulating layer being cracked will become more serious, as the capacity of the rotary machine becomes larger, because the cross-sectional area of the winding conductors and hence the weight thereof are increased as a function of the capacity of the rotary machine and the abrasive relative movement between the conductors and the insulating layer would thus occur under correspondingly increased centrifugal force.
In an attempt to obviate the difficulties described above, there have been already proposed the use of an insulating layer having great thermal and mechanical strength which is, for example, made of glass fibers impregnated with a synthetic resin in a predetermined configuration so as to be suitably disposed between the stacked conductors and the inner walls of the slot. Since such type of a shaped insulation material or layer and in general those having high thermal and mechanical strength are difficult to be bent or folded without incurring deterioration in the mechanical and electrical properties, it is common practice to provide a set of insulating members each formed in a U-like channel bar and adapted to be combined with each other to completely surround the conductors as mounted in each slot.
The insulating layer of the latter structure has proven to be better than the former conventional insulators. However, careful experiments carried out by the inventors have shown that cracks are still produced at some locations such as corner portions of the U-like insulating layer interposed between the securing wedge and the underlying winding conductor upon long running of the rotary machine. More in particular, some distortion stress (remnant stress) as produced during the forming work of the insulation material into the U-like channel bar will inevitably remain at corner portions thereof. Thus, in the operation in which a centrifugal force in the range of 600 to 800 kg/cm.sup.2 is produced, for example, in the case of a turbine generator having rated output on the order of 500 MVA, the corner portions of the upper U-like insulating layer will be subjected to great stresses or abrasive effect due to thermal expansion and/or contractions in the longitudinal and transversal directions of the winding conductor as well as complicated deformation of the conductors and will eventually be cracked. Of course, it can be readily contemplated to increase the thickness of the U-like insulating layer in order to obtain a more rigid structure. However, this requires correspondingly increased depth and width of the winding slot, which in turn will result in an increased size of the rotor and hence of the rotary machine.
Although the function of the insulating layer is simply to prevent the occurrence of a short-circuit between the field winding and the rotor core, difficult problems are involved which have been desired to be solved in respect of the design or construction thereof, as will be understood from the above description.